It is often desirable to drive an element in a machine along a screw which provides accurate positional repeatability and constant drag torque control. Typical applications for such a device are for use in data printers and x-y tables which are used as peripheral equipment in the computer industry.
An example of such a positioning device may be found in U.S. Pat. No. 4,131,031, a previous invention of the patentees which, while it has met with considerable commercial success, has some inherent disadvantages which the present invention overcomes. In the invention of the U.S. Pat. No. 4,131,031, two portions of an axially-split nut are retained in the radial direction by a spacer retaining means and a spring is used to apply longitudinal or axial biasing forces which push the internal threads of the split nut against the flanks of the threads of the driving screw. In order to maintain intimate contact and to avoid backlash, it is necessary to utilize stiff springs which result in high system torque. The higher the system torque, the more expensive the motor needed to drive the system. Also with high system torque, a high degree of friction is created between the threads of the backlash nut and the screw threads which creates more wear on the threads and eventually results in less stiffness in the system. As the stiffness becomes lower, the poorer the repeatability of the system. The apparatus of the present invention has been found to overcome the aforegoing deficiencies in the U.S. Pat. No. 4,131,031 while at the same time, retaining some of the desirable features such as ease in manufacture and low cost.